The disclosure of U.S. Pat. Nos. 8,487,459 and 8,723,355, owned by the assignee of this patent application, are hereby incorporated by reference in their entirety.
A WEC buoy (hereafter, also referred to as a “WEC”) includes a payload which requires a substantial amount of electric power for its operation. The payload is comprised of multiple different “loads” including numerous pieces of electric/electronic equipment and electro-mechanical devices such as, for example, radar and/or sonar systems, various wave and/or climate sensors, communication systems and/or relays, various control systems and associated elements, and a braking system. In order to power and operate the payload, each WEC buoy includes a power-take-off system (PTO) for generating (electric) power in response to wave motion of the body of water containing the buoy, and the generated power can then be used to operate the payload.
A WEC buoy and its associated equipment must remain deployed and operational for long periods of time regardless of the climatic conditions, while still providing substantially constant electrical power to the payload. These climatic conditions can vary greatly, from a calm sea (characterized by low amplitude waves) when little, if any, power is generated, to “storm” conditions, characterized by the amplitude of the waves reaching extreme values, when it may become necessary to “lock-up” the buoy, by means of its braking system, to prevent its destruction (ensure its survivability). Under all operating conditions, stored energy must not be depleted, which would render the payload non-operational.
A WEC buoy in which the present invention would be used may include; (a) a float and a spar which move relative to each other in response to the waves; (b) a power-take-off system (PTO) coupled between the float and spar to convert their relative motion into electrical energy; and (c) a brake system to selectively inhibit the relative motion between the float and the spar, for example, under storm conditions. When relative motion is not inhibited, the PTO, must be able to produce electrical energy reliably and efficiently under widely varying conditions.
In the past, PTOs employed in WEC buoys have utilized a rack and pinion mechanism to convert the bobbing movements of the float into rotary motion to drive an electrical generator. That is, the rack was coupled to receive the movement of the float, driving the pinion into rotation, and the pinion's rotary motion was then coupled to drive a shaft of the electric generator. However, the extremely rough and unpredictable movements encountered by the rack, including substantial, unpredictable lateral movement, made its continuous reliable meshing with the pinion problematic. In addition, significant friction exhibited by the mechanism not only resulted in undesirable wear, but made the energy conversion very inefficient (typically, less than 80% efficiency). Energy used in overcoming friction is not available to produce electricity.
Broadly, it is an object of the present invention to provide a PTO for a WEC buoy with improved efficiency in converting wave energy to electricity. It is also an object of the present invention to improve the durability, reliability and endurance of a PTO for a WEC buoy, making it particularly attractive for use in unattended, autonomously operated WEC buoys.
In accordance with one aspect of the present invention, a PTO for use in a WEC buoy employs a meshing nut and screw shaft assembly for motion conversion. That is, the motion of the float is coupled to drive the nut along the screw shaft on which it is mounted. The linear movement of the nut along the shaft causes the shaft to rotate and this rotational motion is then coupled to rotate an electrical generator. Preferably, the nut and screw shaft comprise a ball screw assembly in which a nut with an internal spiral groove mounts a shaft with a corresponding spiral groove, and circulating ball bearings within the grooves form the interface between the nut and shaft. A PTO of this construction can exhibit in excess of 95% efficiency, high reliability, and no appreciable wear during its lifetime of use.
In accordance with another aspect of the present invention, a PTO for use in a WEC buoy employs a meshing nut and screw shaft assembly for motion conversion and a drive tube concentric with the screw shaft has a first end fixed to the nut and a second end constructed to be coupled to receive the motion of the float. The driving force to the nut is thereby provided co-axially to the shaft, as opposed to previous embodiments that the drive force was eccentric to the ball screw. Preferably, the screw shaft, drive tube and generator are mounted as a unit to a two-axis gimbal, so that the nut can be driven co-axially to the screw shaft even when the float imparts lateral motion to the drive tube.